1. Field of the Invention
The present invention relates to a light source and, particularly, to a high intensity light source for use in a projection system.
The invention has particular application to light sources for use with spatial light modulator devices.
A spatial light modulator is an optical device which is controllable so as to modulate an incident light beam. Colour spatial light modulators are known in which beams of different colours are reflected from different spatial light modulator devices, each driven in accordance with a different video signal. The coloured modulated beams are then combined to form a single projected colour display.
2. Description of the Prior Art
One known type of light modulators is an active matrix device, comprising a matrix of individually addressed pixels in the form of light valves or modulators. A system having an array of liquid crystal light modulators is described in EP-A-0401912 in which light is variably transmittable through each element of the array which in turn modulates the amplitude of the light passing through that element.
A tiltable mirror device is disclosed in U.S. Pat. No. 4,856,863, which shows devices having miniature mirrored elements, wherein each element includes electrodes and is arranged to be electro-statically deflectable between two positions, the extent of deflection being controllable by the extent of the applied electrostatic potential. Such devices may also be operated in a binary mode, in which each mirrored element is arranged to switch between two discrete deflection states, so as to reflect incident light into either a first position or a second position, so as to represent light or dark in the final output beam.
Using a tiltable mirror device, as each mirrored element is individually addressable, a two dimensional image can be reproduced by exposing the array to an incident light beam, modulating the incident beam by controlling the individual mirror devices from a video signal and collating the beam reflected in a particular direction. The small size of the mirrored elements, together with their very fast switching times, allows the elements to be operated at video rates, facilitating the display of a real time video image.
The incident beam does not scan the array, in the way in which an electron beam scans in a cathode ray tube, but is arranged to illuminate the entire device. Thus, given that a high intensity output is desirable in a projection system, it is desirable to illuminate the device with a high intensity beam. A system of this type is described in international application WO91/15843, assigned to the present assignee.
Not only must a high intensity beam be supplied to the array of devices, but the beam must be substantially uniform and be generated by a compact light generating means, in order that the overall dimension of the projection device be manageable.
A compact high intensity light source is manufactured by ILC Technology Inc. of California USA consisting of a compact xenon arc lamp, arranged to operate with an input power supply of one killowatt to produce a two inch diameter beam.
Much of the radiated energy produced in such a device cannot be used, either because it cannot be focused into the beam or because it does not lie within the visible spectrum. In order to increase the power output, it would be possible to increase the power input. There is, however, a limit to the amount of power which may be supplied to the device, due to electrode wear, overheating and general safety constraints.
It is an object of the present invention to provide an improved light source. It is a further object of the present invention to provide an improved light source having improved light efficiency. It is a further object of the present invention to provide an improved light source with improved heat dissipation. Furthermore, it is an object of the present invention to provide an improved light source arranged to operate with an increased power input.
Another problem with known light sources is that the beam tends not to be uniform across its diameter. In particular, a central hole is often present due to the presence of the arc generating electrodes which obscure light as it is reflected back from an associated reflector.
When operating over larger distances, beam divergence ensures that the central hole is filled in and, when the light source is used as a searchlamp for example, no noticeable hole is present and the distribution of light across the beam width appears substantially Gauslan. However, at short distances, such as those present in projection systems, the presence of the hole is noticeable and the distribution of light across the beam diameter is noticably non-isotropic.
According to an aspect of the present invention, there is provided a light source comprising light generating means, and a reflector arranged to reflect light from the generating means into a directional light beam, wherein said beam is not uniform due to part of the reflector being obscured by the generating means, characterised by secondary reflection means arranged to direct part of the reflected beam so as to compensate for the obscured region.
Preferably, the secondary reflector consists of an annular reflector arranged to reflect peripheral light from the edge of a beam in a direction normal to the axis of propagation and a conical reflector centrally positioned so as to redirect said normal beam into the direction of propagation.
Another problem with known arc light sources is that, although they may operate at reasonable voltages during steady state operation, a very high voltage may be required to initiate discharge. This creates problems in that additional equipment must be provided for generating the initial start up voltage and, furthermore, measures must be taken to protect other equipment from the effects of such a high voltage.
According to another aspect of the invention, there is provided a light source comprising an anode, a cathode and an electrical power source for creating an arc between said anode and cathode, characterised in that said anode and cathode are enclosed within a local atmosphere and a radio active source is introduced to ionise the enclosure, thereby reducing the level of an initial voltage required to initiate the arc.
A suitable radio active source may be included in or on the anode or the cathode. Alternatively, a radio active source may be included in the material of an associated reflector if said reflector is enclosed within the arc source.
Other aspects of the invention will become apparent from the description of the embodiments, hereinafter described.